In normal extraocular muscle, our objective is the serial analysis of mammalian extraocular muscle for the purpose of (1) discriminating morphologically distinct populations of muscle fibers in terms of their internal cell structure, innervation, and histochemical profile, (2) ascertaining topographical organizations of such muscle fiber populations within a given extraocular muscle, and (3) comparing the various extraocular muscles in terms of their respective fiber populations and their organization. Emphasis will be placed on morphometric quantification of the mitochondria, sarcoplasmic reticulum, and myofibrillar organization, whereby (a) respective fiber types may be defined and differentiated, and (b) the systematic morphological variation which occurs among certain fiber types may be demonstrated. Sequential histochemical analysis will be correlated with the ultrastructural findings. In abnormal extraocular muscle, our objectives are (a) the analysis of biopsy specimens from cases of neuromuscular disorders which are associated with various syndromes of interest, with emphasis on obtaining insights with regard to myopathic versus neurogenic primacy, and (b) the serial analysis of muscles from myotonic mice, as well as from rats in which myotonia is drug-induced, with emphasis on relating the onset and reversibility of the syndrome to structural modifications, and (c) the serial analysis of denervated extraocular muscle in rat.